In the prior art zig zag sewing machines such as the industrial lock stitch buttonhole sewing machine shown in the Patent No. 961,936 of the Federal Republic of Germany, an arm shaft is rotatably mounted on an arm of a machine frame and carries a handwheel member at one end portion. The arm shaft performs a clockwise rotation as seen from the handwheel member end. Due to such rotation of the arm shaft, a reciprocating motion is applied to a needle bar. Also a lateral movement in a plane perpendicular to the axis of the arm shaft is applied to the needle bar.
A bed shaft is mounted on a bed of the machine frame parallel with said arm shaft and extending from a position corresponding to the reciprocating path of the needle towards the same side as said arm shaft. A loop taker having a thread seizing rotary hook is mounted to the bed shaft adjacent the needle path. As shown in FIG. 1a, said loop taker is rotated synchronously with and in the same direction as said arm shaft in the ratio of 2:1 for each revolution of the arm shaft, with the thread seizing rotary hook of said loop taker being positioned on the opposite side of said bed shaft in regard to the reciprocating path of said needle.
This type of loop taker has been used for a long time in industrial lock stitch buttonhole sewing machines or high speed zig zag sewing machines. For construction reasons, the thread seizing rotary hook of the loop taker has been positioned on the opposite side of the bed shaft in regard to the reciprocating path of the needle, so that the upper and lower threads are arranged on the same side of the needle and the stitches are necessarily formed to perfect stitches. Lock stitches may be classified into perfect stitches and hitch stitches, of which the former stitches are preferred by many because of their excellent appearance and possibility of ornamental stitching.
However, in this type of sewing machine, the upward movement of the needle and the encounter distance of the loop taker and the needle may be changed considerably depending on whether the needle bar is raised while it is at the center of the oscillation amplitude (FIG. 1b) or while it is at the left hand portion of the oscillation amplitude (FIGS. 2b and 3). Said upward movement of the needle is construed as meaning the upward movement wherein the needle vertically moves from the lowest portion to the position at which the thread seizing rotary hook encounters the center line of the needle, and said encounter distance is considered as meaning the distance between the thread seizing rotary hook of the loop taker and the top end of a thread aperture of the needle at which the thread seizing rotary hook encounters the center line of the needle.
Hence it is not possible to provide uniform stitches without skipped stitches and with the predetermined encounter distance in each point of the oscillation amplitude of the needle bar.
For instance, as shown in FIG. 1a, with the radius R1 of the locus of rotation of a crank pin mounted on a crank member being equal to 17.5 mm, the radius R2 of the locus of rotation of the rotary hook of the loop taker being equal to 20.9 mm, the distance L1 between the axes of rotation of the arm shaft and the loop taker being equal to 196.5 mm, the distance L2 between the upper and lower pivot axes of a crank rod being equal to 45 mm and the distance L3 between the lower pivot axis of the crank rod and the upper end of the thread aperature of the needle being equal to 117.6 mm, the distance l1 between the upper end of the thread aperture and the locus of rotation of the thread seizing rotary hook is equal to 4.5 mm when the needle has reached its lower dead point with the needle bar being then at the center of the oscillation amplitude (FIG. 1a). Assuming that this sewing machine is so designed that the rotary hook encounters the needle with an encounter distance l2 of 2 mm when the needle has been raised 2.5 mm, with the arm shaft being swung through an angle .theta.1 equivalent to around 26.38.degree., the upward movement of the needle from the lower dead point and the encounter distance as obtained when the needle bar is swung towards left or right with amplitude l3 equal to 3 mm, as shown in FIGS. 2a, 2b and 3, are as shown in Table 1 below.
TABLE 1 ______________________________________ Needle Bar Position Left Right Hand Center Hand ______________________________________ Upward Movement Magnitude (mm) 1.38 2.5 3.92 Encounter distance (mm) 2.83 2.0 0.92 ______________________________________
When the needle bar is raised while it is at the left hand position of the oscillational amplitude as shown in FIGS. 2a and 2b, the needle is displaced 3 mm to the left of the central position and hence the thread seizing rotary hook of the loop taker encounters the needle when the arm shaft has rotated through an angle .theta. 2 which is smaller than said angle .theta. 1 and is equal to .theta. 1 less a/2, i.e., equal to about 22.26.degree.. Thus, the upward movement of the needle is reduced by 1.12 mm and the encounter distance l2 increased by 0.83 mm as compared to the case of FIG. 1b wherein the needle bar is raised while it is at the center position. The angle a is the angle that the line connecting the pivot or rotational axes of the arm shaft and loop taker makes with the line connecting the rotational axis of the loop taker and the extremity of the thread seizing rotary hook. When the loop taker is rotated through this angle a, the arm shaft is rotated through a half angle, i.e., a/2.
On the other hand, when the needle bar is raised while it is towards the right hand side of the oscillation amplitude, the needle is displaced 3 mm towards the right and hence the rotary hook encounters the needle when the arm shaft has rotated through .theta. 3 which is larger than .theta. 1 and is equal to .theta. 1 plus a/2, or about 30.51.degree.. Thus, the upward movement of the needle is increased by 1.42 mm and the encounter distance is reduced by 1.71 mm as compared to the case of FIG. 1b wherein the needle bar is raised while it is at the center position. Thus, towards the left and right hand sides of the oscillation amplitude, the upper thread loop carried by the needle is not seized satisfactorily by the thread seizing rotary hook, thus resulting in stitch skipping or other stitching defects.
In addition, in this type of prior art sewing machine, the threading operation through the needle must be performed from the side of the needle through which the arm shaft and the bed shaft extend, i.e., from the rear side of the needle, due to the mechanical disposition of the loop taker. This is a laborious threading operation.
In order to overcome such defect, in a further prior art industrial lock stitch buttonhole sewing machine, the arm shaft is arranged to rotate counterclockwise when looking from the handwheel member end. In this case, since the rotational directions of the arm shaft and the loop taker are opposite to each other, the state of encounter of the thread seizing rotary hook of the loop taker with the needle may be improved at the left and right hand portions of the lateral needle motion, as will be appreciated from the subsequent description of FIGS. 4 and 5.
However, such arrangement has given rise to a further problem. Since the rotational direction of the arm shaft is now reversed, if it is desired to use the conventional link type thread take up mechanism without any design change, the disposition of the link type thread take up mechanism as viewed from the handwheel member end of the arm shaft must be reversed. This means that the thread aperture of the thread take up lever is positioned towards the left when looking from the handwheel member end of the arm shaft. This gives rise to serious problems since most users are right handed and the threading operation through the thread aperture of the thread take up lever becomes difficult if the thread aperture is positioned towards the left. In addition, the problem associated with threading through the needle still remains.
The thread take up lever may be arranged towards the right by using, for instance, a cam type thread take up mechanism. However, the cam type thread take up mechanism is not suitable for high speed stitching and is rather noisy in operation, whereas the conventional link type thread take up mechanism lends itself to high speed stitching and moreover is not noisy in operation.
In order to overcome these difficulties associated with the thread take up mechanism, the present inventors have devised, in a sewing machine in which the arm shaft is rotated counterclockwise when viewed from the handwheel member end, an improved lock type thread take up mechanism in which a link member has been added and in which the thread aperture of the thread take up lever is provided towards the right when viewed from said handwheel member end. However, such arrangement still has a disadvantage since the addition of the link member lowers the durability of the thread take up mechanism under high speed operation and deteriorates the thread volume curve although the threading operation through the thread take up lever may thereby be facilitated. Moreover, the aforesaid problem concerned with threading the needle may not be solved by this arrangement.
In view of the foregoing, the present invention provides a solution to all these problems inherent in the prior art.